1. Field of the Invention
The present invention relates to a method for forming a surface relief microstructure, especially an optically variable image (an optically variable device) on a paper substrate, a paper product obtainable using the method and an apparatus for forming a surface relief microstructure on a paper substrate. Microstructures, such as holograms may be replicated rapidly and with accuracy on a paper substrate by using the method and the apparatus of the present invention.
2. Description of the Background
JP06122848 discloses a printing method, comprising intaglio printing, an ink is cured by irradiating with electron beam from the back side of the paper printed immediately after the intaglio printing, and using the ink without applying a releasing agent on the surface of the impression.
WO2000053423A1 relates to a method and apparatus for applying discrete area holograms or other optical devices directly onto documents or other substrates in a continuous process analogous to the operation of a printing press. The method comprises: (i) inserting a supply of untreated substrates into the vacuum chamber while at substantially atmospheric pressure; (ii) reducing pressure within the chamber to a level significantly less than atmospheric pressure; (iii) applying casting resin to discrete areas of the substrate; (iv) holding a micro-grove pattern of a transfer surface against a surface of the resin in the discrete substrate areas; (v) curing the resin while the transfer surface is being held against the resin; (vi) separating the transfer surface from the cured resin, thereby to retain the micro-groove pattern in the surface of the casting resin; (vii) applying a coating of optical material to the resin surface micro-grooves by means of a technique that is normally carried out in a vacuum, the coating being of a type that allows light to be reflected from the resin surface micro-groove pattern and which is substantially limited to the discrete areas of the substrate; (viii) adjusting the pressure within the chamber to a level of substantially atmospheric pressure; and (ix) removing the treated substrates from the chamber. The curing of the resin is done by electron beam radiation.
WO2005051675 is directed to a method for forming a holographic diffraction grating on a substrate comprising the steps of: a) applying a curable compound to at least a portion of the substrate; b) contacting at least a portion of the curable compound with diffraction grating forming means; c) curing the curable compound and d) depositing a metallic ink on at least a portion of the cured compound.
One disadvantage of the above method is that if the web material and the embossing shim are opaque to ultraviolet light, then the irradiation will not be effective from the web side or the embossing shim side.
EP0338378A1 relates to a method of treating sheet material in a continuous process, comprising the steps of:    printing a visual pattern on at least one side of said sheet material in a first area thereof, thereafter applying a resin in liquid form to a second area of said at least one side of said sheet material, said second area being separate from said first area,    holding against said resin a mold of a surface relief pattern in the form of a light diffraction pattern, thereby causing a surface of said resin to conform with said pattern,    directing actinic radiation through said sheet material to said resin in a manner to harden said resin while being carried by said mold,    separating said mold from the hardened resin, thereby to leave the hardened resin in place on said sheet material with the surface relief pattern contained therein, and    coating substantially only the hardened resin in said discrete area with a reflective material in a manner to follow the surface relief pattern,    whereby said sheet material is treated with both conventional printing and a light diffraction pattern in sequential steps of a continuous process.
According to EP0338378A1 the type of radiation that is used depends primarily upon the particular resin formulation and the nature of the sheet material. For sheet material of paper or other opaque substances, electron beam radiation is preferable. For optically transparent sheet material, either totally or partially, ultraviolet radiation can alternatively be used.
EP540455A1 describes a process for preparing printed sheets with optical effects, said sheets comprising a ply of plastics material worked as a lens through which motifs provided therebehind are viewed, characterized in that said sheets (2) are made from non-plastic absorbent material and in a first step they are printed, at least on one surface thereof, by any conventional system (3) with the pertinent motifs or illustrations; in a second step there is applied over the printed surface a resin (6), thermoplastic lacquer or other transparent material which wholly or partially impregnates the surface of the sheet (2), after which the engraving (7-8) which will produce the said optical effects is carried out in a third step on the impregnated area with heat and pressure.
According to FIG. 8 of EP540450A1, after being printed, the paper 2 is fed, while resting on the pertinent roll 11, under the device 5 which applies a resin or varnish 6 thereto. This material, which may be polymerized by ultraviolet rays, impregnates the surface of the paper, which is then fed, resin-coated, to a calender 18 which will apply thereto the engraving producing the optical effects, the resin-coated paper being wrapped on the periphery thereof and accompanying it in part of its rotation until being released from the calender to be fed thereafter to the shears 10 which will cut it into already engraved unit sheets 9.
For curing the calender has the component roll 7′ and the peripheral die plate 8′ transparent, the former being preferably of glass and the latter of polyester, with an ultraviolet ray source 20 being suitably mounted in the interior of the said roll 7′ (FIG. 10). Said rays are projected by the source against the resin-coated surface of the paper 2 during its part rotation with the calender 18.
While according to EP540450A1 several ultraviolet ray sources may also be installed outside the calender 18 (FIG. 9), and acts on the paper 2 from behind, EP540450A1 fails to disclose details concerning the ultraviolet ray source, the UV varnish and photoinitiators.
EP1720079A1 describes a process of producing a coloured hologram, comprising the steps of: preparing a lacquer composition comprising a UV/EB-hardening acrylic resin and at least one pigment, said resin providing instant hardening upon irradiation; applying said lacquer composition to selected areas (6) of a flexible support (S) by means of a rotary printing machine; shaping the applied lacquer to impart it a relief that forms a hologram (8); and irradiating said shaped areas with a UV light/EB radiation (10).
The resin is cured by means of an UV lamp 10 positioned adjacent to master roller 7; if substrate S is paper or a similar non-transparent material, the UV lamp is located where the substrate leaves roller 7, on the side of the holograms 8, and is referred to as lamp 10a in FIG. 2 and in FIG. 4.
In WO94/18609 the curing of the radiation curable media is achieved by the use of a UV source that is located within the bore of a hollow quartz cylinder that is carrying the microstructure relief image to be molded. In one embodiment the relief image is formed in a polymer sleeve that has been placed or cast on the outer surface of the quartz cylinder. The polymer sleeve Is substantially transparent to the UV radiation that is used to cure the cast radiation curable resin. In a further embodiment the microstructure relief image is cast on the cylinder using UV curable resin system.
WO2006032493A2 suggests to use at least two UV sources; one located within the bore of the hollow cylinder and the other located beneath the cylinder and proximate to the back surface of the web as it passes around the hollow cylinder. In this arrangement the uncured resin when in contact with the surface relief microstructure of the cylinder is irradiated from above and below through the transparent substrate. It is possible with the present arrangement to utilize two additional UV sources outside of the hollow cylinder in the printing station. The benefit of additional UV irradiation at the contact point between the uncured resin and the surface relief microstructure on the hollow cylinder is that the resin may be cured faster and more thoroughly when in contact with the hollow cylinder surface relief ensuring high image quality and faster web speeds. This arrangement results in maximum UV irradiation in the contact region between the two nip rollers.
WO2008076785A1 discloses a method of making a decorated package comprising providing a material substrate having an inner and an outer surface, the outer surface to form the outer surface of a carton, coating up to about 100% of the outer surface with a radiation curable coating containing a particulate metal, curing the radiation curable coating containing the particulate metal coating by contacting the radiation curable coating containing the particulate metal coating with radiation, applying zero to one or more ink containing coatings to a substantial portion of the radiation curable coating containing the particulate metal, excluding an area that is to contain a hologram, curing the one or more ink containing coatings, applying a substantially transparent radiation curable coating to the surface of the one or more ink containing coatings, and contacting the substantially transparent radiation curable coating with at least one transparent shim containing a negative of a hologram image in an area not having one or more ink containing coatings, and at least partially curing the substantially transparent coating while the at least one shim is in contact with the substantially transparent coating to form the hologram image in the material substrate.
The primary system for UV curing is based on acrylate polymers and monomers and cured through free radical polymerization. Useful photoinitiators include benzoin derivatives, benzil ketals, acetophenone derivatives and benzophenone.
US2009056858A1 relates to a method for obtaining holograms and/or optical effects on a laminar material, said method comprising:    superimposing and pressing a die provided with an original micro-embossed pattern of a hologram, or corresponding to a configuration that can provide an optical effect, on a lacquer or varnish layer applied on a laminar substrate and curing the lacquer layer on said laminar substrates;    providing the die in the form of a laminar material on which the pattern is configured;    dynamically superimposing said lacquer layer applied to the laminar substrate to said die on a support roller, and dynamically pressing the lacquer layer applied to the laminar substrate against the die on said support roller by means of a pressure roller.
Curing comprises radiating the lacquer layer through the laminar substrate by means of an ultraviolet radiation lamp only in the event that the laminar substrate is transparent or translucent.
EP2159055A2 relates to a method which includes creating a predetermined pattern on an embossing substrate, applying an ink to a print substrate, applying the embossing substrate to the ink wherein the embossing substrate imprints the predetermined pattern into the ink, and curing, via a radiation source, the ink such that an imprint of the predetermined pattern is embossed in the ink.
It is emphasized in EP2159055A2 that when using a light based radiation source, such as UV curing source, the embossing substrate should be made from a material that is transparent to UV radiation. Reference is also made to WO00/30854, JP08036352A, JP07052597A, JP06166170A, WO01/30562 and WO2008061930.
The so-called transparent shims possess several disadvantages. Quartz is not robust enough and leads to a slow process. In addition, transparent shims (belts, or sleeves) can be only used a few times due to ageing under UV-light (polymer shims) and printing in register is very difficult.
In view of the above, there exists a need for systems and methods for printing microstructures (surface relief structures) on a paper substrate that fully incorporates surface relief technologies into mainstream printing applications such as secure documents, flexible and rigid packaging, labels, and printed forms.
There also exists a need for systems and methods for printing microstructures on a paper substrate that allows embossing or casting of the surface relief, and metallizing of the surface relief using a conventional printing system such as flexography, rotogravure, offset printing, silkscreen printing, digital printing, and ink jet printing.
Surprisingly, it has been found that a curable composition (varnish) applied on a paper substrate and embossed with a microstructure can be cured through paper when embossing is done.